This slate-pencil sea urchin would have been about 50 mm across. It had probably just died and been washed ashore - still had most of its short thick primary spines and smaller needle like secondary spines.
The primary photo is of the top side (aboral) and the second is of the underside (oral) with the mouth in the middle. Just visible in pic 4 is a triangular white tooth-like structure in the centre which is part of a complex dental apparatus called the "Aristotle's Lantern".
The shorter spines were a deep red and the thicker ones were paler, some with deep red ridges. It is possible that a few long ones were broken at this stage.
Spotted on the rocky ocean shore -low tide mark on the south coast. (Phillip Island)